Ionic liquids that have relatively low viscosity and melting point and are represented by an imidazolium system have been reported in many publications so far. However, the ionic liquids reported so far are lacking in stability because they are low in reducing stability and narrow in potential window, and many ionic liquids have disadvantage such as difficulty in applying to an electrolyte for electric power storage devices. Furthermore, among the ionic liquids having relatively low melting point, some ionic liquids are considered to be lacking in stability because they have a low thermal decomposition temperature. (See Patent Document 1 and Non-Patent Documents 1 and 2)
As an ionic liquid stable over a wide temperature range, there has been reported an ionic liquid that is formed using as a cation a nitrogen atom-containing onium represented by an ammonium cation. However, an ionic liquid having an ammonium cation has relatively high melting point and viscosity, and only a few have such a structure that provides a low viscosity liquid at around room temperature. (See Patent Document 2, Patent Document 3, and Non-Patent Documents 3 to 6)
In other words, the fact that there are only a few ionic liquids that are stably in a liquid state over a wide temperature range and excellent in electrochemical stability has posed a large barrier when trying to use an ionic liquid is used for lithium secondary batteries, electrical double layer capacitors, fuel cells, dye-sensitized solar cells, or as an electrolyte, an electrolytic solution, or an additive for electric power storage devices.    Patent Document 1: Japanese Patent Laid-Open Publication No. 2001-517205,    Patent Document 2: International Publication No. WO02/076924,    Patent Document 3: Japanese Patent Laid-Open Publication No. 2003-331918,    Non-Patent Document 1: Hagiwara Rika, Electrochemistry, 70, No. 2, 130 (2002),    Non-Patent Document 2: Y. Katayama, S. Dan, T. Miura and T. Kishi, Journal of The Electrochemical Society, 148 (2), C102-C105 (2001),    Non-Patent Document 3: Matsumoto Hajime and Miyazaki Yoshinori, Yoyuen Oyobi Kouonkagaku, 44, 7 (2001),    Non-Patent Document 4: H. Matsumoto, M. Yanagida, K. Tanimoto, M. Nomura, Y. Kitagawa and Y. Miyazaki, Chem. Lett, 8, 922 (2000),    Non-Patent Document 5: D. R. MacFarlane, J. Sun, J. Golding, P. Meakin and M. Forsyth, Electrochimica Acta, 45, 1271 (2000), and    Non-Patent Document 6: Doulas R. MacFarlane, Jake Golding, Stewart Forsyth, Maria Forsyth and Glen B. Deacon, Chem. Commun., 1430 (2001).